This invention relates to the preparation of synthetic organic polymers. More particularly, this invention relates to the preparation of polyesters from hydroxycarboxylic acids or from polyfunctional alcohols and carboxylic acids.
Condensation polymers derived from di- and polyfunctional alcohols include, among others, saturated and unsaturated polyesters, polyester-amides, polyurethanes and polyacetals. Polyesters are a particularly preferred class of condensation polymers because their properties make them suitable for a variety of end-use applications including textile fibers, films, coatings and engineering plastics. By judiciously selecting monomers and polymerization conditions, it is possible to optimize those properties desired for a particular end use.
A conventional method for preparing polyesters and other condensation polymers derived from hydroxyl-containing monomers is by direct esterification, whereby the appropriate monomers, usually polyhydric alcohols or phenols and aliphatic, cycloaliphatic or aromatic polycarboxylic acids are reacted while removing the water formed as a by-product of the esterification reaction. The conditions required to obtain polyesters of the desired molecular weight are disclosed in numerous patents and texts. Specific procedures for preparing representative polyesters and other condensation polymers are described in a text entitled: "Preparative Methods of Polymer Chemistry" by W. R. Sorenson and T. W. Campbell (Interscience Publishers, New York, 1961). All of the procedures for polyesters disclosed in this publication require maintaining the reagents in a molten state throughout the polymerization reaction. In many instances the water produced as a by-product of the reaction is continuously removed during the polymerization.
It is known that the reaction between an alcohol and a carboxylic acid or ester involves an equilibrium that can be represented by the equation EQU ROH+R'COOR".revreaction.R'COOR+R"OH.
In the foregoing equation R and R' are hydrocarbyl groups and R" represents a hydrocarbyl group or a hydrogen atom. When R" is hydrogen, the reaction is referred to as a "direct esterification". The reaction with a compound wherein R" is a hydrocarbyl group is referred to as a "transesterification", since the alcohol residue present on the initial ester, R'COOR", is replaced by the residue of the alcohol ROH. When R" is hydrocarbyl it is preferably methyl or ethyl to facilitate removal of the free alcohol, R"OH, that is produced as a by-product of the reaction. Irrespective of whether R" is hydrocarbyl or hydrogen, the prior art teaches that removal of the R"OH by-product is essential to avoid hydrolysis or alcoholysis of the desired ester. This requirement also applies to polyesterification reactions, the only difference being that the carboxylic acid and the alcohol represented by ROH in the foregoing equation are polyfunctional.
It is also well known that the molecular weight of a polyester formed by direct esterification is determined to a large extent by the efficiency with which the by-product is removed from the reaction mixture. If the desired molecular weight is relatively low, the water or alcohol can be evaporated or distilled under atmospheric pressure from a reaction mixture wherein the reagents are in molten form and at a temperature of from about 150.degree. to 250.degree. C. This process can often be facilitated if an inert gas is passed through the reactor. To achieve the higher molecular weights desired for textile fiber production, it is usually necessary to either completely remove the water or alcohol under reduced pressure or employ an organic solvent that forms an azeotropic mixture with these by-products.
One disadvantage inherent in the foregoing prior art teachings is that the energy input required to remove all but trace amounts of the R"OH by-product substantially increases the cost of manufacturing polyesters.
A second disadvantage associated with preparing condensation polymers by conventional bulk and solution polymerization techniques is that the high viscosity exhibited by these products makes them difficult to transfer and process.
One method for avoiding the problems associated with the manufacture and processing of relatively high molecular weight polymers in molten or solubilized form is to employ a technique known as emulsion polymerization whereby one or more monomers are reacted in an aqueous medium containing a catalyst and, usually, a surfactant. The final polymer is obtained as an aqueous emulsion or latex exhibiting a relatively low viscosity, sometimes approaching that of water. Heretofore emulsion polymerization employing water as the continuous phase has been employed substantially exclusively for the polymerization of ethylenically unsaturated compounds in the presence of free radical sources, such as organic peroxides. Since the presence of even small amounts of water during condensation polymerizations involving polyhydric alcohols and polycarboxylic acids has been shown to substantially reduce the molecular weight of the resultant polyester, emulsion polymerization in aqueous media has heretofore not been considered a practical means for preparing condensation polymers in general, and particularly polyesters and other polymers derived from polyfunctional alcohols.
It is known to prepare certain types of condensation polymers, particularly polyamides, by interfacial polymerization. In accordance with this method, an aqueous phase containing a solubilized or emulsified diamine such as hexamethylene diamine, usually in the form of the corresponding sodium salt, is combined with a water-immiscible organic liquid such as methylene chloride containing a solubilized diacyl halide such as sebacoyl dichloride. A relatively rapid formation of solid polymer occurs at the interface between the two liquid layers. If the reaction is to proceed to completion, the polymer must be continuously removed from the area of the interface by stirring the reaction mixture or by withdrawing the polymer from the interfacial region as the reaction progresses.
Polyfunctional carboxylic acids or their corresponding anhydrides are not used for interfacial polymerization because these compounds do not have the required high level of reaction rate exhibited by the corresponding acyl halides. The acyl halides react so rapidly that no catalyst is required. By comparison, the reaction of polyfunctional carboxylic acids with di- or polyhydric alcohols requires a polycondensation catalyst to achieve a useful reaction rate.
Since both interfacial polymerization, as it has been applied to the formation of polyamides, and emulsion polymerization employ an aqueous phase, this relatively large amount of water would be expected to displace the equilibrium of the polyesterification reaction in the direction of degradation of any polymer formed to the corresponding polyfunctional carboxylic acid and alcohol. It is therefore not obvious to employ either of these techniques as a means for preparing commercially useful polyesters.
Surprisingly it has now been found that polyesters wherein the average number of repeating units per molecule is as high as 20 or more can be prepared by emulsion polymerization in aqueous media and in the presence of specified polycondensation catalysts.